1. Field of the Invention
The present invention relates to a sensor, and more particularly, it relates to a sensor having a diaphragm.
2. Description of the Background Art
A sensor such as a sonic sensor having a diaphragm is known in general, as disclosed in Japanese Patent Laying-Open No. 60-500841 (1985), for example.
The sonic sensor proposed in the aforementioned Japanese Patent Laying-Open No. 60-500841 comprises a vibratory diaphragm formed by a silicon film having a small thickness and a fixed electrode plate of a metal opposed to the diaphragm. When the diaphragm vibrates by a sound pressure in this sonic sensor, the distance between the diaphragm and the fixed electrode plate changes due to this vibration. Thus, the electrostatic capacitances of capacitors formed by the diaphragm and the fixed electrode so change that the sonic sensor converts sounds to electric signals by outputting changes in the quantities of charges in the diaphragm and the fixed electrode plate resulting from the changes of the electrostatic capacitances as the electric signals.
Assuming that ε represents the dielectric constant between the diaphragm and the fixed electrode plate when the diaphragm and the fixed electrode plate have the same surface areas S, electrostatic capacitances C1 and C2 of two capacitors at different distances d1 and d2 (d1>d2) between the diaphragm and the fixed electrode plate can be expressed as follows:C1=εS/d1  (1)C2=εS/d2  (2)
When the average distances between the diaphragm and the fixed electrode plate are sonically reduced by an average displacement δd in the respective capacitors, electrostatic capacitances C1a and C2a of the capacitors can be expressed as follows:C1a=εS/(d1−δd)  (3)C2a=εS/(d2−δd)  (4)
Therefore, the displacements δC1 and δC2 of the capacitors can be expressed as follows:δC1=C1−C1a=εSδd/d1(d1−δd)  (5)δC2=C2−C2a=εSδd/d2(d2−δd)  (6)
d1>d2 from the assumption, and hence δC1<δC2.
As understood from the expressions (5) and (6), therefore, the change δC2 of the electrostatic capacitance in the capacitor having the smaller distance (d2) between the diaphragm and the fixed electrode plate exceeds the change δC1 of the other capacitor when the distances between the diaphragm and the fixed electrode plate are reduced by the same value δd in these capacitors. Consequently, the electric signal output on the basis of the capacitor having the smaller distance (d2) between the diaphragm and the fixed electrode plate more largely changes, thereby improving sensitivity of the sonic sensor.
In the sonic sensor according to the aforementioned Japanese Patent Laying-Open No. 60-500841, however, the diaphragm formed by the silicon film, wholly made of the same material (silicon) with the same thickness, entirely exhibits the same elastic modulus. The outer periphery and the central portion of the diaphragm entirely having the same elastic modulus are substantially uniformly bent in a convex manner due to vibration resulting from a sound pressure, to increase the displacement (quantity of bending) of the central portion toward the fixed electrode plate. Therefore, it is difficult to reduce the distances between the diaphragm and the fixed electrode plate. In the sonic sensor according to the aforementioned Japanese Patent Laying-Open No. 60-500841, therefore, it is disadvantageously difficult to improve the sensitivity.
In this regard, U.S. Pat. No. 5,146,435, for example, proposes a sonic sensor reducing the displacement (quantity of bending) of a central portion of a diaphragm by suppressing bending of the central portion. The sonic sensor proposed in the aforementioned U.S. Pat. No. 5,146,435 comprises the diaphragm formed by a silicon film, a spring film of silicon integrally provided on part of the outer periphery of the diaphragm and a fixed electrode plate opposed to the diaphragm. In this sonic sensor, the spring film of silicon has a smaller thickness than the diaphragm of silicon, to be easily sonically vibrated (bent). On the other hand, the diaphragm having a larger thickness than the spring film is hard to bend. When the sonic sensor according to the aforementioned U.S. Pat. No. 5,146,435 receives a sound, therefore, the spring film is vibrated (bent) by a sound pressure. Following this vibration of the spring film, the diaphragm is vibrated along with the spring film without much bending. Therefore, the distance between the diaphragm and the fixed electrode plate, which may be set only in consideration of vibration (bending) of the spring film, can be set to a small level. Consequently, sensitivity can be improved in the sonic sensor according to the aforementioned U.S. Pat. No. 5,146,435.
In the sonic sensor disclosed in the aforementioned U.S. Pat. No. 5,146,435, however, the spring film, formed with the thickness smaller than that of the diaphragm to be easily vibrated, is disadvantageously easy to break.